Ch 13 Pt 2 animal viruses
a viral titer can be estimated using
quantal assay
in latent infections
the viral genome remains silent within a host cell, yet can reactivate to cause a productive infection
not all enveloped viruses have cytoplasmic membrane derived envelopes, some obtain....
their envelope from the membrane of an organelle such as the golgi apparatus or the rough endoplasmic recticulum -they do this by budding into the organelle, form there they are transported in vesicles to the outside of the cell -budding may not destroy the cell because membranes can be repaired after the viral particles exit
Cell culture/tissue culture
animal cells- grown in a liquid medium contained in special screw caped flasks- are used as host cells for the virus culture -animal cells provided with the proper nutrients can divide repeatedly -animal cells grow more slowly than most bacteria -dividing no faster than 24 hrs
these mutations lead to
anitgenic variation and allow some RNA viruses to adapt to selective pressures. -Influenza viruses for ex exhibit a type of genetic variation called Antigenic Drift
a generalized infection cycle of animal viruses can be viewed as a five-step process
attachment genome entry synthesis assembly release
for ex:
consider a virus strain that has RNA segments "a" through "e" and a different strain that has segments "A" through "E" -all the new viral particles being generated must have a total of 5 segments, but as these can have different combinations of capitalized and regular letters aBCDE, abcDE, AbcDE, and so on
today cell culture or tissue culture is used to
cultivate most animal viruses
certain viruses cause an infected cell to form a
distinct region called an INCLUSION BODY, the site of viral replication -the position of an inclusion body in a cell depends on the type of virus
many viruses can be detected by their
effect on cells in cell cultures -a virus generated in a cell culture often causes distinct morphological alterations in infected cells, called a CYTOPATHIC EFFECT. -the host cells may change shape, detach from the surface, or lyse -infected cells may fuse into a giant multinuclear cell (syncytium), a mechanism of viral spread
non-enveloped viruses only enter host cells through
endocytosis
enveloped viruses enter by 1 of 2 mechanisms
fusion with the host membrane or endocytosis
to study viruses they must be
grown in the laboratory in an appropriate host
Antigenic Drift
-occurs as mutations accumulate in genes encoding key viral surface proteins that are recognized by the immune system -because of these changes, a person whose immune response protected him or her against influenza virus one year, may not be protected against the strain that circulates the next year.
certain viruses cause red blood cells to agglutinate (clump). this is called
Hemagglutination -only visible at high concentrations or viruses and can be used to determine only the relative concentration of viral particles
the viral proteins that are made are sometimes synthesized as
a poly-protein that is subsequently split by viral proteases into individual proteins
most enveloped viruses are released from the host by
budding, a process where the virus acquires its envelope -before budding occurs, virally encoded protein spikes insert into specific regions of the host cells membrane -matrix protein accumulates on the inside surface of those same regions. -nucleocapsids are then extruded from the cell at those regions, becoming covered with a layer of matrix protein and lipid envelope in the process
persistent infections
can continue for years, or even the life of the host with or without symptoms
Non enveloped viruses which have no lipid bilayer
cannot fuse to host membranes to enter cells
Chronic infections
characterized by the continuous production of low levels of viral particles -in some cases the infected cell survives and slowly releases viral particles -in other cases, the infected cell lyses, but only a small proportion of cells is infected at any given time, resulting in a low number of viral particles being continuously released.
acute infections
characterized by the sudden onset of symptoms of a relatively short duration
two general types of persistent infections can be identified
chronic and latent *these categories may overlap in an actual infection
double stranded RNA viruses
have a genome that consists of both a (+) RNA strand and a (-) RNA strand
Acute infections
result in a burst of virions being released from infected host cells -although the virus infected cells often die, the host may survive -this is because the immune system of the animal host may gradually eliminate the virus over a period of days to months -symptoms of the disease result from localized or widespread tissue damage following cell death as well as damage caused by the immune response itself.
one way to obtain animal cells for culture is
to remove tissue from an animal and then process it to get individual cells -these cells can then be grown in a flask with a liquid nutrient medium -cells acquired in this way form primary cultures -a problem with this approach is that normal cells can divide only a limited number of times, even when diluted into a fresh medium, and so new primary cultures must be made -to avoid this problem TUMOR cells are often used in cell culture -these cells multiple indefinitely in vitro, resulting in what is called an established cell line.
animal cells types that make up solid tissue
typically grow as a monolayer, -a single sheet of cells adhering to the bottom of the flask
production of viral particles in an infected cell requires two events
1) expression of viral genes to produce structural and catalytic proteins, such as capsid proteins and any enzymes required for replication 2) synthesis of multiple copies of the viral genome
assembly and maturation
-an important step in the replication of viruses -involves bringing together newly formed viral nucleic acid with capsid proteins and packaging them to form the nucleocapsid -it is a spontaneous self assembly process that occurs when there is an appropriate amoun t of viral nucleic acid and capsid proteins in the host cell -often involves modification of the newly synthesized viral proteins, which may occur within the host cell or after the new virions leave the host cell.
attachment (absorption)
-animal viruses have attachment proteins or spikes on their surfaces -the receptors to which these proteins bind are usually glycoproteins on the host cell cytoplasmic membrane. -often more than one receptor is required for effective attachment
Replication of double-stranded DNA viruses
-follows central dogma of molecular biology -the genome of a double-stranded DNA virus can be referred to as (+/-) to indicate that it has both strands -the (+/-) genomes can be transcribed to produce mRNA. mRNA is transcribed as a + strand. -mRNA is then translated to make proteins -double stranded DNA genome also serves as a template for DNA replication
replication of single stranded (-) RNA virus
-more complicated because (-) RNA strand cannot be translated -it must first be copied into a (+) strand before protein synthesis can occur -this is done by a replicase carried by the virus. -once the (+) RNA strand has been produced, it can be translated to make viral proteins, and is also used as a template for synthesizing new (-) RNA strands -one of the proteins made during translation is replicase, and as the new viral particles are assembled, a molecule of the replicase is packaged along with single stranded (-) DNA
the site of assembly an maturation differs according to the virus
-non enveloped viruses mature fully in the host cell cytoplasm -in the case of enveloped viruses, some maturation steps occur as the virion leaves the host cell -the virus deposits protein into the host cytoplasmic membrane and the assembled nucleocapsid then binds to these regions of the membrane before exiting the host cell *some DNA viruses such as herpes virus does this
quantal assay
-several dilutions of the virus preparation are administered to a number of animals, cells, or chick embryos, depending on the host specificity of the virus
Replication of single stranded DNA viruses
-similar to replication of double stranded DNA viruses, however a complement to the single stranded DNA molecule must be synthesized to generate a double stranded (+/-) DNA molecule -once that is done, the genes can be expressed to produce the encoded proteins
DNA viruses
-these viruses replicate in the nucleus of the host cell and use the host cell machinery for DNA synthesis as well as gene expression -DNA viruses often encode their own DNA polymerase, this allows them to replicate even if the host cell is not actively replicating its own chromosome
replication strategy of viruses can be divided into three categories
-those used by 1) DNA viruses 2) RNA viruses 3) reverse transcribing viruses
double stranded RNA viruses
-uncommon -must also carry their own replicase because the host cell machinery is unable to translate double stranded RNA -the replicase uses the (-) RNA strand of the double stranded RNA molecule as a template to make the (+) strand RNA -this molecule is then translated to make more replicase and the infection cycle can continue
replication of RNA viruses
-vast majority are single stranded and they replicate in the cytoplasm -replication always requires a virally encoded RNA polymerase called replicase -replicase is an RNA dependent RNA polymerase meaning that it is able to synthesize a strand of RNA from an RNA template -recall that the RNA polymerase used in transcription is a DNA dependent RNA polymerase -the virus needs an enzyme that uses an RNA template to make more copies of RNA
HIV has features of both acute and persistent infections
-when a person is first infected with HIV they experience symptoms such as fever, fatigue, swollen lymph nodes, and headache -the immune system soon eliminates most virions and the symptoms subside -however, the DNA copy of the viral genome integrates into the host cell chromosome, resulting in a persistent infection with subsequent symptoms associated with AIDS developing after many years
to be maintained in nature, infectious virions must leave one animal host
and be transmitted to another
since a virion must bind to specific receptors
a particular virus may be able to infect only a single or limited # of cell types and tissues called tissue tropism
the silent viral genome is called
a provirus
relationship btwn viruses and their animal hosts can be divided into two categories of infections
acute and persistent
Release from the host cell
also depends on whether the virion is enveloped or non enveloped
Retroviruses
include the human immunodeficiency virus (HIV), have a (+) strand RNA genome, and carry reverse transcriptase within the virion -after entering the host cell, the reverse transcriptase uses the RNA genome as a template to make one strand of DNA -the complement to that DNA strand is then synthesized to make double stranded DNA, which integrates into the host cell chromosome -once integrated the viral DNA may remain in a latent state or it may be transcribed into RNA which is translated to synthesize viral proteins needed for production of new virions
Process of hemagglutination occurs when
individual viral particles attach to the surface molecules of multiple red blood cells simultaneously, connecting the cells to form an aggregate.
most viruses
infect only a single species (the viral host range) -this accounts for the resistance that some animals have to certain diseases -for ex: dogs don't contract measels from humans
tissue tropism
inside a host the virus may only infect certain tissues
the titer of the virus, or endpoint...
is the dilution at which 50% of the inoculated hosts are infected or killed -this can be reported as either the ID50 (infective dose), or the LD50(lethal dose)
the fact that a provirus cannot be eliminated from the body
means that the disease can ocur even after an extended period without symptoms
Hemmaglutination is measured by
mixing serial dilutions of the viral suspension with a standard amount of red blood cells -the highest dilution showing maximum clumping is the titer of the virus
most DNA viruses
multiply in the nucleus -they enter the nucleus through nuclear pores using viral proteins that have nuclear localization signals
uncoating
nucleic acid separates from its protein coat prior to replication
influenza virus is an example
of a virus with a segmented genome that can undergo reassortment
the mechanism an animal virus uses to enter its host cell depends
on whether the virion is enveloped or non enveloped. -in all cases, the entire virion is taken into the cell -whereas in phages only the nucleic acid enters and the capsid stays outside the bacterium
one group of animal viruses that can agglutinate red blood cells is the
orthomyxoviruses
one of the most precise methods for determining the concentration of animal viruses in a sample is the
plaque assay -a monolayer of cultured tissue cells is the host -clear zones surrounded by uninfected cells are counted to determine the viral titer.
Replicases lack
proofreading ability and as a result they make more mistakes than DNA polymerase, generating mutations during replication
if two different strains of a segmented virus infect the same cell
reassortment can occur -meaning that during replication, new viral particles are made that have combinations of genome segments from the initial infecting strains
Reverse transcribing viruses encode the enzyme
reverse transcriptase -an RNA-dependent DNA polymerase, which synthesizes DNA from an RNA template -this runs counter to the central dogma of molecular biology in that an RNA molecule is used as a template to make DNA
white blood cells grow as
single cells in suspension
of the single stranded RNA viruses
some types have a (+) RNA strand genome, meaning that the genome also serves as mRNA -others have a (-) RNA strand genome, meaning that the genome is a complement to mRNA
Viruses that enter by endocytosis
take advantage of receptor mediated endocytosis -the viral particles bind to the receptors that normally trigger and facilitate the process, causing the cell to take them up. -after the virion is taken into the cell, the viral envelope fuses with the membrane of the endosome releasing the nucleocapsid in the cytoplasm.
the viral proteases that split the poly proteins can be useful
targets for antiviral medications
the important aspect of either type of persistent infection is
the continuous production of infectious viral particles, often in the absence of disease symptoms -consequently a person can transmit the virus to others even in the absence of symptoms
uncoating may occur simulataneously with
the entry of the virion into the cell, or after the final intracellular destination for viral replication has been reached
whether latent or productive, once integrated
the genome can direct a productive infection or remain in a latent state -once the DNA copy is made it can't be eliminated from the cell
Non enveloped viruses are released when
the host cell dies -many viruses trigger a normal cellular process called apoptosis, or programmed cell death, prior to the release of the viral particles -the immune response of an animal, which is also directed towards eliminating the virus, can also lead to the same process. -the virions released from the dead cells may then invade any healthy cells in the area
in fusion
the lipid envelope of the virion fuses with the cytoplasmic membrane of the host cell after the virion attaches to the host cell receptor.
As a result of fusion
the nucleocapsid is released directly into the cytoplasm
Antigenic shift
the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains.
when the RNA virus has a single strand (+) RNA genome
the replication strategy is simple because the genome functions as mRNA -the viral RNA can immediately bind to host cell ribosomes and translated to make proteins -one of the proteins encoded is a replicase, which is needed to make more copies of (+) RNA strand. It can only do this by first using the (+) RNA strand (the viral genome that entered the cell) as a template to make complementary (-) strands -the (-) strands act as templates to produce additional (+) RNA strands
the replication strategy for RNA viruses depends on
the type of viral genome -some have a single stranded genome and others have a double stranded genome.
cytopathic effects are often characteristic for a particular virus..
they are useful to scientists studying and identifying viruses
the newly synthesized (+) RNA strands serve two purposes
they can be translated to make more viral proteins or they can be packaged into new virions being formed.
